Method and apparatus for dynamic roll compensation of streamer for marine geophysical surveying

ABSTRACT

Included are methods and apparatus for marine geophysical surveying. One embodiment of the presently-disclosed solution relates to a method for instantaneous roll compensation of vectorised motion data originating from a fixed-mount geophysical sensor during a marine seismic survey. A streamer is towed behind a survey vessel in a body of water. The streamer includes a plurality of geophysical sensors and a plurality of orientation sensor packages, and each orientation sensor package comprises a magnetometer. Vectorised geophysical data is acquired using the plurality of geophysical sensors, while orientation data is acquired by the plurality of orientation sensor packages. The orientation data is used to determine an instantaneous roll angle of the streamer at different positions on the streamer. The vectorised geophysical data is adjusted to compensate for the instantaneous roll angle of the streamer at different positions on the streamer. Other embodiments and features are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 62/433,305, filed Dec. 13, 2016, the entire disclosure of which isincorporated herein by reference.

BACKGROUND

Embodiments relate generally to the field of marine geophysicalsurveying. Techniques for marine geophysical surveying include seismicsurveying and electromagnetic surveying, in which geophysical data maybe collected from below the Earth's surface. Marine geophysicalsurveying has applications in mineral and energy exploration andproduction and may be used to help identify locations ofhydrocarbon-bearing formations.

Certain types of marine geophysical surveying, including seismic andelectromagnetic surveying, may include using a survey vessel to tow anenergy source at selected depths—typically above the seafloor—in a bodyof water. One or more streamers may also be towed in the water at theselected depths by the same or a different survey vessel. The streamersare typically cables that include a plurality of geophysical sensorsdisposed thereon at spaced apart locations along the length of thecable. Some geophysical surveys locate the geophysical sensors on oceanbottom cables or nodes in addition to, or instead of, streamers. Thegeophysical sensors may be configured to generate a signal that isrelated to a parameter being measured by the geophysical sensor.

At selected times during a marine geophysical survey, an energy sourcemay be actuated to generate, for example, seismic or electromagneticenergy that travels downwardly into the subsurface formation. Energythat interacts with interfaces, generally at the boundaries betweenlayers of subsurface formations, may be returned toward the surface anddetected by the geophysical sensors on the streamers. The detectedenergy may be used to infer certain properties of the subsurfaceformation, such as structure, mineral composition and fluid content,thereby providing information useful in the recovery of hydrocarbons.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present invention and should not be used to limit or define theinvention.

FIG. 1 illustrates an example marine geophysical survey system with astreamer having a sensor package in accordance with example embodiments.

FIG. 2 illustrates the coordinate system for a particular sensor packagein accordance with example embodiments.

FIG. 3 illustrates an example portion of a streamer having a sensorpackage and a streamer rotation device in accordance with exampleembodiments.

FIG. 4 illustrates a sensor package disposed on a circuit board inaccordance with example embodiments.

FIG. 5A illustrates raw primary orientation sensor data in accordancewith example embodiments.

FIG. 5B illustrates raw complementary orientation sensor data inaccordance with example embodiments.

FIG. 6A illustrates a scatterplot of the Y-Z trajectory of the rawprimary orientation sensor data in accordance with an embodiment of theinvention.

FIG. 6B illustrates a scatterplot of the Y-Z trajectory of the rawcomplementary orientation sensor data for example embodiments.

FIG. 7A is a graph of the magnitude of y-z local-position componentsfrom DC-coupled accelerometer data as a function of time.

FIG. 7B is a histogram of the magnitudes in FIG. 7A.

FIG. 7C is a graph of the y-z local-position trajectory from theaccelerometer data. As depicted, the y-z position trajectory confirmsthe roll motion of the seismic streamer.

FIG. 7D is a graph of the roll angle as a function of time from theaccelerometer data.

FIG. 8 is a flow chart of a method of dynamic roll compensation of avector sensor in a streamer environment using a magnetometer inaccordance with an embodiment of the invention.

FIG. 9 is graph showing examples of roll angle extracted from low-passfiltered magnetometer data with cut-off frequencies at 2 Hz, 4 Hz and 6Hz in accordance with an embodiment of the invention.

FIG. 10 is a graph showing that the roll-compensated fixed-mount sensordata corresponds very well with the gimbaled sensor data in accordancewith an embodiment of the invention.

DETAILED DESCRIPTION

The use of fix-mounted geophysical sensors in a streamer environment hassome advantages over the use of gimbal-mounted geophysical sensors. Inparticular, fix-mounted sensors have simpler mounting technology that ismore reliable and less costly than gimbaled sensors.

The removal of the gimbal poses a challenge, however, in that theorientation of the geophysical sensor needs to be determined to ahigh-degree of accuracy. This is especially true for the horizontalcomponent due to the direction of the incoming wavefield. Furthermore,in order to avoid any instantaneous roll effects in the geophysicaldata, the compensation of roll angle should be done dynamically, ratherthan in a static manner.

One technique that addresses these problems uses a DC-coupledaccelerometer as an orientation sensor. This technique is problematic,however, in that the data from a DC-coupled accelerometer may be heavilycontaminated by translational acceleration noise, particularly at thestreamer head and tail, and during turns of the survey vessel.

The present disclosure provides a solution for instantaneous rollcompensation of vectorised motion data originating from a fixed-mountgeophysical sensor. The solution is advantageously less sensitive tocontamination by translational acceleration noise.

One embodiment of the presently-disclosed solution relates to a methodfor instantaneous roll compensation of vectorised motion dataoriginating from a fixed-mount geophysical sensor during a marineseismic survey. A streamer is towed behind a survey vessel in a body ofwater. The streamer includes a plurality of geophysical sensors and aplurality of orientation sensor packages, and each orientation sensorpackage comprises a magnetometer. Vectorised geophysical data isacquired using the plurality of geophysical sensors, while orientationdata is acquired by the plurality of orientation sensor packages. Theorientation data is used to determine an instantaneous roll angle of thestreamer at different positions on the streamer. The vectorisedgeophysical data is adjusted to compensate for the instantaneous rollangle of the streamer at different positions on the streamer.

Another embodiment of the presently-disclosed solution relates to anapparatus for performing a marine seismic survey. The apparatus includesa streamer to be towed behind a survey vessel in a body of water duringthe marine seismic survey. A plurality of geophysical sensors on thestreamer acquires geophysical data during the marine seismic survey. Inaddition, a plurality of orientation sensor packages on the streameracquires orientation data during the marine seismic survey, where eachorientation sensor package comprises a magnetometer. A recording systemreceives and processes the geophysical and orientation data. Therecording system uses the orientation data to determine an instantaneousroll angle of the streamer at different positions on the streamer, andthe recording system adjusts the geophysical data to compensate for theinstantaneous roll angle of the streamer at different positions on thestreamer.

FIG. 1 illustrates a marine geophysical survey system 5 in accordancewith an embodiment of the present invention. In the illustratedembodiment, the marine geophysical survey system 5 may include a surveyvessel 10 that moves along the surface of a body of water 15, such as alake or ocean. The survey vessel 10 may include thereon equipment, showngenerally at 20 and referred to for convenience as a “recording system.”The recording system 20 typically includes devices for navigating thesurvey vessel 10, such as global positioning system (“GPS”) receivers,devices for actuating one or more energy sources 25, and devicesfor-recording signals generated by geophysical sensors 30.

As illustrated, the survey vessel 10 or a different vessel (not shown)can tow energy sources 25, for example, using source cable 35. Theenergy sources 25 may be towed above the water bottom. The energysources 25 may be a selectively actuable energy source suitable formarine geophysical surveying, including seismic air guns, water guns,vibrators, electromagnetic field transmitters, or arrays of suchdevices. In some embodiments, seismic energy and/or electromagneticenergy may originate from the energy sources 25. As the energy isemitted by the energy sources 25, it travels downwardly through the bodyof water 15 and subsurface formations (not shown) below the waterbottom. It should be noted that, while the present example shows only asingle energy source 25, the invention is applicable to any number ofenergy sources 25 towed by survey vessel 10 or any other vessel.

The marine geophysical survey system 5 may include a plurality ofstreamers 40 towed by the survey vessel 10 (or another vessel) with eachof the streamers 40 including the geophysical sensors 30 at spaced apartlocations. The streamers 40 may be towed above the water bottom. Thestreamers 40 may be spaced apart laterally, vertically, or bothlaterally and vertically. “Lateral” or “laterally,” in the presentcontext, means transverse to the direction of the motion of the surveyvessel 10. The streamers 40 may be coupled at their forward end (withrespect to the direction of movement of the survey vessel 10) tocorresponding lead-in lines 45. While not shown, each of the lead-inlines 45 may be deployed from the survey vessel 10 by a winch or othersimilar spooling device, for example, that can be used to control thedeployed length of the lead-in lines 45. It should be noted that, whilethe present example shows only four streamers 40, the invention isapplicable to any number of streamers 40 towed by survey vessel 10 orany other vessel. For example, in some embodiments, eight or morestreamers 40 may be towed by survey vessel 10, while in otherembodiments, as many as twenty-six or more streamers 40 may be towed bysurvey vessel 10.

The geophysical sensors 30 may be of various types or combinationsthereof. Examples of geophysical sensors include seismic sensors such asgeophones, hydrophones, or accelerometers; or electromagnetic fieldsensors such as electrodes or magnetometers. In an exemplaryimplementation, each of the geophysical sensors 30 may be a dual-sensordevice. The dual-sensor device may include, for example, a pressuresensor and a vertical velocity sensor. By way of example, thegeophysical sensors 30 may generate response signals, such as electricalor optical signals, in response to detecting energy emitted from theenergy sources 25 after the energy has interacted with the rockformations (not shown) below the water bottom. Signals generated by thegeophysical sensors 30 may be communicated to the recording system 20.In accordance with an embodiment of the invention, the signals generatedby the geophysical sensors 30 and communicated to the recording system20 comprise vectorised seismic data in that the signals include bothmagnitude and directional information.

In an exemplary implementation, a geophysical data product indicative ofcertain properties of the subsurface rock may be produced from thedetected energy. The geophysical data product may include processedseismic or electromagnetic geophysical data and may be stored on anon-transitory, tangible computer-readable medium.

The streamers 40 may further include streamer rotation devices 50. Asillustrated, the streamer rotation devices 50 may be disposed on thestreamers 40 at spaced apart locations. In some embodiments, thestreamer rotation devices 50 may have a spacing of about 200 meters toabout 400 meters. Each streamer rotation device 50 may providerotational torque to cause its rotation, and likewise rotation of theportion of the streamer 40 adjacent to the streamer rotation device 50.The streamer rotation devices 50 may also be used, for example, in theregulation of the streamer depth and streamer lateral positioning. Inaddition to regulation of depth and streamer lateral position, thestreamer rotation devices 50 may also contain devices that output rollcount and/or force data. In some embodiments, existing lateral force anddepth (LFD) control devices may be used as streamer rotational devices.The LFD control devices may be any of a variety of different devicessuitable for changing streamer orientation and/or position, including“birds” having variable-incidence wings. In one example, streamerrotation devices 50 may include variable incidence wings that arerotatably fixed onto the streamer. Examples of suitable streamerrotation devices 50 are disclosed in U.S. Pat. No. 6,525,992 and U.S.Pat. No. 6,932,017.

The streamers 40 may further comprise sensor package 55. The sensorpackage 55 has an output that is representative of, or can be processedto determine, the orientation of the corresponding streamer 40 at thesensor package 55 with respect to the streamer's longitudinal axis. Forexample, the sensor package 55 may be used to measure rotation of thecorresponding streamer 40 about its longitudinal axis. In someembodiments, the sensor package 55 may comprise at least one primaryorientation sensor 41 which may measure angles of slope and/orinclination of the corresponding streamer 40 (or any geophysicalsurveying equipment to which it may be coupled to) with respect toEarth's gravity. The primary orientation sensor 41 may measure the rollangle of the corresponding streamer 40 about its longitudinal axis. Theroll angle θ is shown on FIG. 2. The local coordination system (x, y, z)for a particular sensor package 55 is shown on FIG. 2. In theillustration of FIG. 2, the roll angle θ is the angle of rotation aboutlongitudinal axis 110 of the streamer 40 between the global Z-axis (Z)and the local z-axis (z). Examples of suitable primary orientationsensors 41 include accelerometers which may be DC-coupled to the Earth'sgravity, which are commonly referred to as DC-coupled accelerometers.DC-coupled accelerometers may be considered to be coupled to the Earth'sgravity because DC-coupled accelerometer measurements include thegravity component of acceleration. The accelerometer may be used, forexample, to measure acceleration. A micro-electrical mechanical systems(MEMS) accelerometer is an example accelerometer sufficient for use as aprimary orientation sensor 41. The primary orientation sensor 41 maymeasure the projection of the gravity vector along the sensing axis. Insome embodiments, a 2-axis primary orientation sensor 41 may provide ameasurement of linear acceleration along the x- and y-axes. In someembodiments, a 3-axis primary orientation sensor 41 may provide ameasurement of linear acceleration along the x-, y-, and z-axes. Becausethe force of gravity is known and always acts towards Earth's center,the accelerometer outputs may be processed to compute the roll angle.

Sensor package 55 may further comprise a complementary orientationsensor 42 (FIG. 4) which may be DC-coupled, for example, to measure theEarth's magnetic field. Complementary orientation sensor 42 may measureangles of slope and/or inclination of the corresponding streamer 40 (orany geophysical surveying equipment to which it may be coupled to) withrespect to Earth's magnetic field. In some embodiments, thecomplementary orientation sensor 42 may measure the roll angle of thecorresponding streamer 40 about its longitudinal axis. The roll angle θis shown on FIG. 2. The local coordination system (x, y, z) for aparticular sensor package 55 is shown on FIG. 2. In the illustration onFIG. 2, the roll angle θ is the angle of rotation about longitudinalaxis 110 of the streamer 40 between the global Z-axis (Z) and the localz-axis (z). Examples of suitable complementary orientation sensors 42may include vector magnetometers which may be DC-coupled to the Earth'smagnetic field. Specific examples may include fluxgate andmagnetoresistive magnetometers. The magnetoresistive magnetometer maycomprise a permalloy magnetometer or may be a tunneling magnetoresistivemagnetometer. The complementary orientation sensor 42 may measure theinclination, azimuth, and total intensity of the Earth's magnetic field.In some embodiments, the complementary orientation sensor 42 may be usedto measure the magnetic field and magnetic inclination and azimuth at apoint, and this information may be compared to the heading of the surveyvessel 10/sensor package 55. From this information, the roll angle θ maybe measured relative to gravity.

As discussed above, some embodiments of primary orientation sensor 41(e.g., a DC-coupled accelerometer) may produce inaccurate orientationdata (e.g., excess noise) when overlaid linear acceleration is present(e.g., when turning). Further, some embodiments of primary orientationsensor 41 may be sensitive to vibration (e.g., vibration which may occurat the front of the streamer 40). Complementary orientation sensor 42may be used to provide accurate orientation data during turns and/or insituations in which vibration (e.g., vibration large enough to interferewith primary orientation sensor 41) is present. Complementaryorientation sensor 42 (e.g., a DC-coupled magnetometer) may produceinaccurate orientation data (e.g., excess noise) when electricalinterference is present. Further, embodiments of complementaryorientation sensor 42 may not be used when the magnetic inclination isnear 0°. Primary orientation sensor 41 may be used to provide accurateorientation data when electrical interference is present and/or insituations where the magnetic inclination is near 0°. Therefore, primaryorientation sensor 41 and complementary orientation sensor 42 may beused in conjunction to produce orientation data that is more accurate inmore circumstances than using either orientation sensor alone.

Measurements by the complementary orientation sensor may be used inplace of, or in supplement to, measurements by the primary orientationsensor to provide accurate orientation data during turns. Measurementsby the complementary orientation sensor is used in place of, or insupplement to, measurements by the primary orientation sensor to provideaccurate orientation data when vibrations larger than a threshold aredetected in the measurements by the primary orientation sensor.

Measurements by the primary orientation sensor is used in place of, orin supplement to, measurements by the complementary orientation sensorto provide accurate orientation data when electrical interference ispresent and when magnetic inclination of the magnetometer is near zero.Measurements by the primary orientation sensor is used in place of, orin supplement to, measurements by the complementary orientation sensorto provide accurate orientation data when electrical interference abovea threshold is present in the measurements by the complementaryorientation sensor.

The sensor package 55 may be rigidly mounted to the correspondingstreamer 40 so that its output represents streamer roll at its currentlocations. In some embodiments, the sensor package 55 may not beco-located with the streamer rotation devices 50. In some embodiments,the sensor package 55 may not be co-located with any geophysical sensors30. Alternatively, the sensor package 55 may be co-located with thestreamer rotation devices 50 and/or the sensor package 55 may beco-located with any geophysical sensors 30. As used herein, sensors areconsidered co-located when the sensors are fixed to the same mechanicalmount or otherwise cannot move relative to one another. In furtheralternative embodiments, the sensor package 55 may be disposed on orabout the streamer rotation devices 50 and/or the geophysical sensors 30in place of, or in addition to, mounting of the sensor packages 55 onthe streamers 40. In some embodiments, complementary orientation sensor42 may not be a component of every sensor package 55. For example,complementary orientation sensor 42 may not be a component of a sensorpackage when vibration is not present at a magnitude to affect primaryorientation sensor 41 (e.g., 500 m to 1 km from survey vessel 10) orwhen other complementary orientation sensors 42 provide enough accurateorientation data during turns to eliminate the need for placement of acomplementary orientation sensor in every sensor package 55. Scatteredusage of complementary orientation sensors 42 as necessary may reduceexpenses and preparation time as there would be fewer complementaryorientation sensors 42 to purchase, install, and calibrate. From ameasurement perspective, it may be desirable to have a less densedistribution of sensor packages 55 than geophysical sensors 30, as theoperational requirements are likely to be non-overlapping.

The sensor packages 55 may be spaced along the length of the streamers40 as desired. In some embodiments, the sensor packages 55 may have aspacing of from about 2 meters to about 50 meters on the streamers 40 ora particular section thereof. In particular embodiments, the sensorpackages 55 may have spacing of about 5 meters to about 10 meters on thestreamers 40 or a particular section thereof. In some embodiments, thesensor packages 55 may have a uniform distribution along the streamers40 or a particular section thereof. For example, the sensor packages 55may be uniformly distributed over a streamer section having a length offrom about 75 meters to about 150 meters in some embodiments. In someembodiments, at least 10 sensor packages 55 and up to 20 or more sensorpackages 55 may be distributed over the streamer 40 section. Sensorpackages 55 may be distributed on streamer 40 at a spacing in a range ofbetween, and including any of, about 2 to about 14 meters. For example,sensor packages 55 may be distributed in streamer 40 at a spacing ofabout 2 meters, about 4 meters, about 6 meters, about 8 meters, about 10meters, about 12 meters, or about 14 meters. In a specific example,sensor packages 55 are distributed on streamer 40 every 6.25 meters.

FIG. 3 shows an example portion of a streamer 40 having a streamerrotation device 50 and sensor package 55. As illustrated, the sensorpackage 55 may also be distributed along the portion of the streamer 40.While not illustrated, one or more geophysical sensors 30 (e.g., shownon FIG. 1) may also be distributed along the portion of the streamer 40.The streamer rotation device 50 may be disposed on the streamer 40. Asillustrated, the streamer rotation device 50 may have wings 65 coupledto a device body 70. While two wings 65 are shown on FIG. 3, embodimentsof the streamer rotation device 50 may comprise more (or less) than twowings 65. In some embodiments, the streamer rotation device 50 may bedisposed inline between adjacent streamer sections 40 a, 40 b. To causerotation, rotational torque may be introduced into the wings 65 toproduce rotation of the streamer rotation device 50. Rotational torquemay be introduced by, for example, introducing a wing rotation (e.g.,clockwise rotation) relation to the wing axis to generate torque on thestreamer 40. As the streamer rotation device 50 rotates, the rotationaltorque may be incrementally reduced to slow the rate of rotation untilthe desired rotation has been achieved. The streamer rotation device 50may further include a local rotation device control system 75. In someembodiments, the local rotation device control system 75 may function tocontrol rotational movement of the streamer rotation device 50.

As illustrated in FIG. 4, sensor package 55 may be disposed on a circuitboard 80 in accordance with example embodiments. Circuit board 80 may bean internal or external component of any geophysical surveying equipment(e.g., streamers, sensors, ocean bottom cables, sources, paravanes,etc.). Circuit board 80 may comprise a processor 85, input/output(“I/O”) interface(s) 90, primary orientation sensor 41, and/orcomplementary orientation sensor 42. Processor 85 may comprise one ormore central processing unit(s) or may be distributed across one or moreprocessors in one or more locations. I/O interface(s) 90 may becommunicatively coupled to processor 85. I/O interface(s) 90 may be anysuitable system for connecting circuit board 80 to a recording system 20(as shown in FIG. 1). A communication link (not shown) may be used toconnect recording system 20 to circuit board 80. Examples of acommunication link include a direct connection, a private network, avirtual private network, a local area network, a wide area network(“WAN”), a wireless communication system, or combinations thereof.

The primary orientation sensor 41 and the complementary orientationsensor 42 may be coupled to processor 85. In some embodiments, theprimary orientation sensor 41 and the complementary orientation sensor42 may be integrated. In alternative embodiments, the primaryorientation sensor 41 and the complementary orientation sensor 42 maynot be integrated. In some embodiments, it may be beneficial if thecomplementary orientation sensor 42 is co-located with the primaryorientation sensor (e.g., fixed to the same circuit board). In someembodiments, it may be beneficial if the complementary orientationsensor 42 is near (e.g., within 10 cm) to the primary orientationsensor. In some embodiments, where the primary orientation sensorcomprises a MEMS accelerometer, it may be beneficial if thecomplementary orientation sensor 42 is also a component of the MEMSdevice, such that the primary orientation sensor 41 and thecomplementary orientations sensor 42 make a single MEMS device. In someembodiments, a gyroscope 86 could be used with or in place of amagnetometer for the MEMS device. With the use of the gyroscope 86, therotational part of the MEMS primary orientation sensor 41 (e.g., anaccelerometer) could be separated from the linear motion. This may inturn enable suppression of the lateral noise seen on a MEMSaccelerometer. However, the gyroscope 86 may be harder to calibrate,since it may lack a stable external reference.

Data processing and analysis software native to recording system 20and/or installed on recording system 20 may be used to analyze the datagenerated by sensor package 55. This procedure may be automated suchthat the analysis happens without the need for operator input orcontrol. Further, the operator may select from several previously inputparameters or may be able to recall previously measured data. The datamay be transferable and/or storable on computer-readable media, such asone or more USB drive(s), if desired.

The effect of overlaid linear acceleration (e.g., turn noise) on asensor package 55 which includes a primary orientation sensor 41 (e.g.,an accelerometer) and a complementary orientation sensor 42 (e.g., amagnetometer) is depicted on FIGS. 5A and 5B. FIG. 5A illustrates rawaccelerometer data, and FIG. 5B illustrates raw magnetometer data. Thisdata was collected during a 540° turn, using a sensor package 55comprising an integrated primary orientation sensor 41 (e.g., theaccelerometer device) and a complementary orientation sensor 42 (e.g.,the magnetometer device) attached to a streamer 40 (e.g., in theconfiguration illustrated on FIG. 1). Such data may be received during aturn or other operation of a streamer 40. The difference between thereadings from the primary orientation sensor 41 and the complementaryorientation sensor 42 is not readily apparent when analyzed through justthe raw data. However, if the raw data is converted to a scatterplot ofthe y-z trajectory of the streamer transversal plane the differencebecomes apparent. FIGS. 6A and 6B illustrate examples of the convertedraw data as the scatterplot of the y-z trajectory of primary orientationsensor 41 and the complementary orientation sensor 42 data. FIG. 6Aillustrates converted raw accelerometer data, and FIG. 6B illustratesconverted raw magnetometer data. The level of overlaid linearacceleration is now readily distinguished when comparing FIGS. 6A and6B. The overlaid linear acceleration present in the primary orientationsensor 41 (e.g., the accelerometer) data is visually represented asblurring and/or other variations in the circular nature of the circleillustrated in FIG. 6A. As shown by FIG. 6B, this blurring is greatlyreduced. As such, the significantly lower noise levels in thecomplementary orientation sensor 42 data may significantly reduce thenoise in the streamer 40 roll angle determination.

Given the local coordinate system (x, y, z) as shown in FIG. 2 for asensor package on a seismic streamer, FIG. 7A is a graph of themagnitude of y-z local-position components from DC-coupled accelerometerdata as a function of time. FIG. 7B is a histogram of the magnitudes inFIG. 7A. As shown in FIGS. 7A and 7B, the magnitude varies about thenormalized value of 1.00 as a function of time. This is consistent witha roll motion of the seismic streamer.

FIG. 7C is a graph of the y-z local-position trajectory from theaccelerometer data. As depicted, the y-z position trajectory confirmsthe roll motion of the seismic streamer.

FIG. 7D is a graph of the roll angle as a function of time from theaccelerometer data. In this example, the roll angle varies as a functionof time within a range from about 135 degrees to about 165 degrees. Thevarying roll angle of the sensor package on a seismic streamer causesinaccuracies in the orientation data obtained by a fixed-mount motionsensor.

FIG. 8 is a flow chart of a method 800 of dynamic roll compensation of avector sensor in a streamer environment using a magnetometer inaccordance with an embodiment of the invention. The method 800 providesinstantaneous roll compensation at in time during a marine seismicsurvey in an advantageous way that is less sensitive to contamination bytranslational acceleration noise.

The right branch of the flow chart pertains to the acquisition ofgeophysical data and includes block 802. The left branch of the flowchart pertains to the acquisition and processing of orientation data andincludes blocks 804, 806 and 808. In accordance with an embodiment ofthe invention, the left and right branches are performed in parallelduring a marine seismic survey.

Per block 802, vectorised geophysical data is acquired during the marinegeophysical survey. The geophysical data is vectorised in that it hasboth magnitude and directional components. For example, a geophonemeasures a velocity that is proportional to the Earth particle velocityat its location. Hence, geophones are vector sensors and may be used tomeasure both a magnitude of Earth motion and a direction of that motion.In contrast, hydrophones measure pressure variations and may be used toobtain scalar (not vectorised) geophysical data. This is because thepressure data does not provide directional information.

Per block 804, orientation data is acquired in parallel with (i.e. atthe same time as) the acquisition of the vectorised geophysical dataduring the marine geophysical survey. During the marine geophysicalsurvey, the survey vessel may travel along a path that includes variousturns so as to cover a desired region. During such turns, theorientation of the geophysical acquisition equipment may vary, resultingin changes in the orientation data.

In accordance with an embodiment of the invention, DC-coupledmagnetometers are used to acquire the orientation data. In addition tothe DC-coupled magnetometers, MEMS accelerometers may also be used. Inan exemplary implementation, the accelerometers may be used as primaryorientation sensors, and the magnetometers may be used as complementaryorientation sensors.

Per block 806, the orientation data from each magnetometer may below-pass filtered. The low-pass filtering may be performed using alow-pass filter with a cut-off frequency of 2 Hertz (Hz), for example.This low-pass filtering removes noise effects in the roll angledetermination.

Per block 808, instantaneous roll angles may be determined for eachmagnetometer. Each magnetometer has a fixed local z-axis and providesmeasurements from which the global Z-axis may be calculated. Hence, asindicated in FIG. 2, the roll angle θ at any instant in time may bedetermined by finding the angle between the global Z-axis and the localz-axis.

Per block 810, the instantaneous roll angles are used to adjust thevectorised geophysical data. In an exemplary implementation, the rollangles for geophysical sensors at locations between magnetometers on astreamer may be determined by a predetermined interpolation functionbased on the roll angles determined at the magnetometers along thestreamer.

Per block 812, the adjusted geophysical data is recorded on acomputer-readable storage medium (or computer-readable storage media).

FIG. 9 is a graph showing examples of roll angle extracted from low-passfiltered magnetometer data with cut-off frequencies at 2 Hz, 4 Hz and 6Hz in accordance with an embodiment of the invention. As shown, usingthe low-pass filtered magnetometer data, the extracted roll angle is asubstantially smooth function of time. Lower cut-off frequencies includeonly lower-frequency data and so result in smoother roll-angledeterminations. In an exemplary implementation, a cut-off frequency of 2Hz may be used for the low-pass filtering.

FIG. 10 is a graph showing that the vertically-aligned roll-compensatedfixed-mount sensor data corresponds very well with the gimbaled sensordata in accordance with an embodiment of the invention. Thevertically-aligned data is obtained by aligning the fixed local z-axisof the magnetometer to the global z-axis. The graph shows amplitude (logscale) as a function of frequency (log scale).

As further shown, the horizontally-aligned roll-compensated fixed-mountsensor data has an amplitude vs. frequency graph that has suppressedpeaks compared to the gimbaled sensor data and also compared to thevertically-aligned roll-compensated fixed-mount sensor data. Thehorizontally-aligned data is obtained by aligning the fixed local y-axisof the magnetometer to the global y-axis. The advantage of using thevertically-aligned data, instead of the horizontally-aligned data, isthus shown by the data in FIG. 10.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Various advantages of the present disclosurehave been described herein, but embodiments may provide some, all, ornone of such advantages, or may provide other advantages.

What is claimed is:
 1. A method of performing a marine seismic survey,the method comprising: towing a streamer behind a survey vessel in abody of water, wherein the streamer includes a plurality of geophysicalsensors and a plurality of orientation sensor packages, wherein eachorientation sensor package comprises a magnetometer; acquiringvectorised geophysical data using the plurality of geophysical sensors;receiving orientation data from the plurality of orientation sensorpackages; using the orientation data to determine an instantaneous rollangle of the streamer at different positions on the streamer; andadjusting the vectorised geophysical data to compensate for theinstantaneous roll angle of the streamer at different positions on thestreamer.
 2. The method of claim 1, wherein acquiring the vectorisedgeophysical data involves actuating an energy source to emit energydownward through the body of water.
 3. The method of claim 1, whereinthe plurality of geophysical sensors includes seismic sensors from thegroup of seismic sensors consisting of geophones, hydrophones,accelerometers, electromagnetic field sensors, and combinations thereof.4. The method of claim 1, wherein the magnetometer comprises acomplementary orientation sensor and measures an inclination andazimuthal angle of the Earth's magnetic field.
 5. The method of claim 4,wherein the orientation sensor package further comprises anaccelerometer that comprises a primary orientation sensor which measuresa projection of gravity along at least one sensing axis.
 6. The methodof claim 5, wherein measurements by the complementary orientation sensoris used in place of, or in supplement to, measurements by the primaryorientation sensor to provide accurate orientation data during turns. 7.The method of claim 5, wherein measurements by the complementaryorientation sensor is used in place of, or in supplement to,measurements by the primary orientation sensor to provide accurateorientation data when vibrations larger than a threshold are detected inthe measurements by the primary orientation sensor.
 8. The method ofclaim 5, wherein measurements by the primary orientation sensor is usedin place of, or in supplement to, measurements by the complementaryorientation sensor to provide accurate orientation data when electricalinterference is present and when magnetic inclination of themagnetometer is near zero.
 9. The method of claim 5, whereinmeasurements by the primary orientation sensor is used in place of, orin supplement to, measurements by the complementary orientation sensorto provide accurate orientation data when electrical interference abovea threshold is present in the measurements by the complementaryorientation sensor.
 10. The method of claim 1 further comprising:filtering the orientation data with a low pass filter to obtainlow-frequency orientation data.
 11. The method of claim 10, wherein thelow-pass filter has a cut-off frequency in a range from 2 Hertz to 6Hertz.
 12. The method of claim 10 further comprising: determining theinstantaneous roll angle at a position on the streamer using thelow-frequency orientation data, wherein the instantaneous roll angle ofthe streamer is determined relative to a vertical direction.
 13. Themethod of claim 1, wherein the vectorised geophysical data has bothmagnitude and directional components, and wherein adjusting thevectorised geophysical data involves changing the directional componentsbased on the instantaneous roll angle of the streamer at the differentpositions on the streamer.
 14. An apparatus for performing a marineseismic survey, the system comprising: a streamer to be towed behind asurvey vessel in a body of water during the marine seismic survey; aplurality of geophysical sensors on the streamer to acquire vectorisedgeophysical data during the marine seismic survey; a plurality oforientation sensor packages on the streamer to acquire orientation dataduring the marine seismic survey, wherein the orientation sensor packagecomprises a magnetometer; a recording system to receive and process thevectorised geophysical and orientation data, wherein the recordingsystem uses the orientation data to determine an instantaneous rollangle of the streamer at different positions on the streamer, andwherein the recording system adjusts the vectorised geophysical data tocompensate for the instantaneous roll angle of the streamer at differentpositions on the streamer.
 15. The apparatus of claim 14 furthercomprising an energy source that is actuated to emit energy downwardthrough the body of water.
 16. The apparatus of claim 14, wherein theplurality of geophysical sensors includes seismic sensors from the groupof seismic sensors consisting of geophones, hydrophones, accelerometers,electromagnetic field sensors, and combinations thereof.
 17. Theapparatus of claim 14, wherein the magnetometer comprises acomplementary orientation sensor that measures an inclination andazimuthal angle of the Earth's magnetic field.
 18. The apparatus ofclaim 17, wherein the orientation sensor package comprises anaccelerometer that comprises a primary orientation sensor which measuresa projection of gravity along at least one sensing axis.
 19. Theapparatus of claim 17, wherein measurements by the complementaryorientation sensor is used in place of, or in supplement to,measurements by the primary orientation sensor to provide accurateorientation data during turns.
 20. The apparatus of claim 17, whereinmeasurements by the complementary orientation sensor is used in placeof, or in supplement to, measurements by the primary orientation sensorto provide accurate orientation data when vibrations larger than athreshold are detected in the measurements by the primary orientationsensor.
 21. The apparatus of claim 17, wherein measurements by theprimary orientation sensor is used in place of, or in supplement to,measurements by the complementary orientation sensor to provide accurateorientation data when electrical interference is present and whenmagnetic inclination of the magnetometer is near zero.
 22. The apparatusof claim 17, wherein measurements by the primary orientation sensor isused in place of, or in supplement to, measurements by the complementaryorientation sensor to provide accurate orientation data when electricalinterference above a threshold is present in the measurements by thecomplementary orientation sensor.
 23. The apparatus of claim 14, whereinthe recording system filters the orientation data with a low pass filterto obtain low-frequency orientation data, and determines theinstantaneous roll angle at a position on the streamer using thelow-frequency orientation data.
 24. The apparatus of claim 19, whereinthe low-pass filter has a cut-off frequency in a range from 2 Hertz to 6Hertz.
 25. The apparatus of claim 14, wherein the vectorised geophysicaldata has both magnitude and directional components, wherein therecording system adjusts the vectorised geophysical data by changing thedirectional components based on the instantaneous roll angle of thestreamer at the different positions on the streamer.
 26. A method ofmanufacturing a geophysical data product, the method comprising:obtaining geophysical and orientation data acquired by towing a streamerbehind a survey vessel in a body of water, wherein the streamer includesa plurality of geophysical sensors and a plurality of orientation sensorpackages; using the orientation data to determine an instantaneous rollangle of the streamer at different positions on the streamer; adjustingthe geophysical data to compensate for the instantaneous roll angle ofthe streamer at different positions on the streamer; and recording theadjusted geophysical data on a computer-readable medium storage.